Semiconductor diode laser arrays are known in the art and are used in a variety of applications in the defense and aerospace fields. Two of the most common uses are illumination and solid-state laser (SSL) pumping, in which the radiation from the diode lasers is used to excite the laser crystal in order to generate light. The SSLs may then be used in a number of configurations and applications, including range finding and target designation. In many SSL applications it is common to operate the laser diode arrays in pulsed, or quasi-continuous wave (QCW) mode. In this mode, the diodes are electrically pumped with a pulse width that is commonly on the order of the upper state lifetime of the laser gain medium. For example, Nd:YAG lasers are typically pumped with pulse widths on the order of 200 μs. This pumping mode leads to efficient laser designs since most of the pump light that is absorbed by the laser crystal may be extracted from the system.
The repetition rate of the diode pumps is also defined by the application. Many range finding applications operate in the 10-30 Hz range, and many direct diode illuminations operate at around 60 Hz to match the frame rate on commercial off the shelf (COTS) camera systems. Quasi-continuous wave QCW diode pumping holds several advantages over CW diode pumping in SSL systems. First, QCW pumping creates a lower average thermal load in the laser gain medium. This simplifies the cooling of the system and also enables higher beam quality lasers due to the reduced thermal lensing effects. Second, QCW pumping allows the diodes to be operated at higher peak power than is possible with CW pumping. This leads to SSL systems with higher peak powers.
Such diode pumps are centered around a laser diode array of several laser diode bars. The laser diode bars are electrically connected together and aligned so that the light path from each of the individual semiconductor laser diode bars is parallel. Typically, laser diode bars are formed on a semiconductor material wafer that is cleaved into individual laser diode bars. The individual diode bars are then loaded into a fixture and high-reflection (HR) and anti-reflection (AR) coatings are deposited on opposing facets of the bars. Spacers are placed between each bar during loading to prevent coating spillover and to maintain the autonomy of each bar. Following facet coating, the bars are unloaded from the fixture, inspected, and placed into carriers until they are needed for packaging into a laser diode array assembly.
For example, a diode bar may be soldered to its own heatsink, which has a coefficient of thermal expansion (CTE) near that of the bar (e.g., ˜6 ppm/K in the case of GaAs). This allows for the use of hard solders such as eutectic AuSn, which minimizes solder creep and promotes a high degree of reliability. The subassembly created when a bar and heatsink are joined is known as a Mounted Bar Assembly (MBA). A number of the MBAs are soldered together such that the associated heatsinks are attached to a ceramic backplane. This array is placed between electrical contacts to create the laser diode array. The electrical contacts also serve as large heatsinks on the end of the array. The bar-to-bar spacing, or pitch, is defined primarily by the thickness of the heatsink and any other spacing between the MBAs. For this type of array created by MBAs, pitch values ranging from 350 μm to 2 mm are fairly common in the industry today, but are much higher than the “brick” style of arrays described below. On the other hand, thermal efficiency in arrays created by MBAs is much better than the “brick” style of arrays described below.
In another packaging arrangement, a substrate (e.g. BeO) has a plurality of spaced apart and generally parallel grooves, each of which receives an individual laser diode bar. A soft solder layer is disposed in each of the grooves and the laser diode bars are soldered in the grooves. Electrical connection between the laser diode bars is accomplished by reflow of the solder layer within the grooves. However, such interconnections are not considered high density because of the wall thickness separating each groove and associated diode bar in the stack.
In many applications, including defense-related applications, it is often advantageous for the laser diode arrays to have a very high output power density. High diode output power densities enable the use of smaller laser crystals and also have a direct impact on the size, weight, and cooling requirements of the resulting laser system. For arrays built using the previously described process, featuring diode bars rated at 200-300 W/bar, the resulting power density is generally in the range of 5-8 kW/cm2. There are several ways to increase the optical power density of a laser diode array. One way is to increase the output power of each of the diode bars that comprises the array. Additionally, optical methods (e.g., interleaving, beam combining) may be used to generate arrays with higher power densities.
Alternatively, the heatsinks or other heat-sinking components between the diode bars may be eliminated, which reduces the bar-to-bar pitch, thereby increasing the optical power density. This design is often referred to as a high-density stack. To build a high-density stack, the individual bars are then loaded into a separate fixture, with solder preforms placed between each bar. The stack of bars is then reflowed to create a “brick” of bars (e.g., 5-10 bars bonded together). In this process, a stack of laser diode bars are directly joined together (each bar bonded directly to adjacent bar) using a solder, such as an AuSn solder. This stack is then attached to a ceramic backplane and electrical contacts in a subsequent soldering step. The resulting array or brick of laser diodes has nominal bar-to-bar pitch of ˜150 μm with pitches of less than 100 μm achievable. This pitch (150 μm) is approximately 40-50% of the smallest industry-standard pitch that may be obtained from other standard packaging methods, including those described above. This leads to optical power densities that are approximately two to three times higher than can be obtained using standard packaging methods.
While this technique produces a very high-density stack, it suffers from potentially damaging or chipping the dielectric anti-reflection and high reflection coatings that have already been applied to facets of each laser diode bar during assembly of the stack. The damage to one bar may result in the loss of the entire array in the production process.
Thus, there is a need for a high-density stack, semiconductor laser diode array that allows direct connection of the individual laser diode bars. There is a further need for a stacked diode array that is fabricated in such a manner to eliminate processing steps. There is also a need for a fabrication method to avoid known failure modes associated with fluxed soft-soldering interconnection methods while at the same time minimizing stress caused by packaging to prevent damage to the laser diode arrays during assembly. There is a further need for minimizing the oxidation of the laser diode bar substrate material when the bars are interconnected. There is also a need for a method of fabrication that minimizes the need for inspection of the bars after applying the anti-reflection and high reflection coatings.